Comment 2 for bug 1686134

reproduced on Windows 10, Inkscape 0.92.1 r15371

likely caused by the fact that the bounding box is obtained from the CLI command Popen('inkscape --query-%s') and this command will not be aware of the fact that the page size has changed.
The slow response time is probably related to the fact that a new instance of Inkscape has been opened.